The interaction of synthetic templates with eukaryotic DNA primase.

The template interaction of calf thymus and human placenta DNA primases has been investigated. Using oligothymidylates, we showed that a template consisting of ten monomeric units was the critical size for interaction with the enzyme. The hydrophobic effect is likely to be a major factor determining template recognition by the DNA primase. The correlation between the template affinity with the enzyme and the octanol-water hydrophobic scale confirms this suggestion. In addition, the electrostatic interaction between the phosphate group of the template and side chains on the enzyme probably increases template affinity. Using methylated poly(dA), we found that the first nucleotide base of the primer should be more hydrophobic than the corresponding nucleotide base of the template. A model for the mechanism of action of DNA primase is suggested on the basis of data presented in this study and previous findings. According to this model, (a) DNA primase binds ten nucleotides of the template; (b) the synthesis of primer up to the formation of a decamer occurs processively and competes with template binding of the enzyme; (c) the conditions under which the nucleotides are incorporated into the RNA product change during the extension of the primer, and these changes are responsible for switching from primase to polymerase activity. The template specificity of DNA primase is likely to play an important role in the initiation and regulation of DNA replication.